1. Field of the Invention
The present invention relates generally to power supplies and, more specifically, the present invention relates to a switching regulator.
2. Background Information
In a switching power supply, a power supply controller is connected in series with a primary winding of a transformer to the high voltage direct current (DC) input derived from alternating current (AC) power line through rectification and filtering. Energy is transferred to the secondary winding in a manner controlled by the power supply controller so as to provide a constant output voltage. Another winding called a feedback or bias winding may be used to provide a feedback signal to the power supply controller. Alternatively, the feedback signal can come through an opto-coupler from a sense circuit at the secondary output. The feedback signal is used to modulate the duty cycle of the power supply controller or used to allow or disallow cycles in order to control the secondary output.
In the event of a short circuit condition, an overload condition (may be caused by either an overload at the output secondary winding or an under voltage condition at the power supply input) or an open in the feedback loop, the power supply controller detects the loss of feedback signal and goes into a mode called “auto-restart.” In the auto-restart mode, the power supply controller tries to start the power supply periodically by delivering power for a period of time, also known as the on-time, (greater than needed for start up) and turns off the power supply for another period of time, also known as the off-time. As long as the fault condition is present, the power supply controller remains in this auto-restart mode limiting the average output power to a safe, low value. When the fault is removed, auto-restart enables the power supply to start automatically.
The operating principle of auto-restart is to reduce the average power delivered to the power supply output during fault condition to a safe level by running the power supply alternately between a first predetermined time period (also known as on-time) and a second predetermined time period (also known as off-time). During on-time power supply is on delivering energy to output while during off-time power supply is off delivering no energy to output. To make auto-restart protection effective, the off-time is set significantly longer than the on-time. When fault condition occurs for a duration shorter than the auto-restart off-time, the power supply will not attempt to restart until the off-time is over resulting in unnecessary long delay before recovery.